1. Field of the Invention
The present invention relates to a method of and apparatus for hardening metal parts by means of a heat treatment process, the process consisting of a heating stage for preheating of the parts to be treated to a temperature in excess of the hardening temperature, a diffusion stage to treat the parts by the diffusion of components from the atmosphere into the parts, a cooling stage to reduce the temperature of the parts to the hardening temperature and a quenching stage to quench said parts.
2. Description of the Prior Art
The above-described hardening techniques are used to heat metal parts to be hardened to their cores, or to heat the surface layers of such metal parts if such parts are large, to a diffusion temperature in excess of the hardening such as a temperature, of 920.degree. C. or any other diffusion temperature and to treat the parts at the diffusion temperature which is maintained for a certain period of time, by carburizing of carbonitriding in a gaseous atmosphere, from which components diffuse into the surface of the parts being treated, whereupon the temperature of the parts is reduced to the hardening temperature, with some diffusion possibly still occurring during the temperature reduction, and then the parts are quenched in a quenching medium. To increase the output of such hardening facilities by means of continuous operation, it is a well-known technique to employ pusher-type plants where the parts so treated are usually combined to form charges consisting of a large number of individual parts which pass through the heating, diffusion and cooling stages of a continuous chamber. If the continuous pusher-type technique described is used, the different chamber zones representing the different treatment stages interact, the temperature and atmosphere transitions between the different zones being gradual. The known technique described does not provide abrupt changes in temperature or atmosphere which would increase the rates of heat or mass transfer. If these rates were increased, energy consumption could be reduced considerably and the metallurgical treatment of parts could be controlled more effectively. If a conventional furnace employing the technique described were to be designed for such abrupt changes in temperature or atmosphere, such continuous furnace chamber would have to be broken up into separate chambers, one such separate chamber being provided for each zone. The locks or vestibules and the gas-tight doors which would be required for separation between such chambers would, however, be very complex and costly in view of the high temperatures at which metal parts are so treated. Further, as in the case of known heat treatment plant designs for batch-type operation, the parts treated in the continuous chamber of conventional continuous installations are usually densely packed, with heat and gaseous atmosphere for treatment having to move to the surface of each part so treated. It is a known drawback of such continuous heat treatment installations that the time required for heating each part and the time required for the process of diffusion is considerably longer than in a case where each part is treated separately, and that the rate of heat transfer to each such part constituting the charge may be different, causing the warping of parts and thus necessitating costly straightening and other post-treatment operations.